Rainfall in South Texas brings new concerns for agriculture

Rainfall has sure changed the landscape from what looked like a desert all summer long to now greening pastures and fields. The results are mostly all good unless you have a sunflower or sesame crop still standing in water needing to be harvested. Rain also brings out pests like weeds and mosquitoes, but we can deal with those. Farmers should also be on the lookout for emerging cotton seedlings, as they need to be destroyed so boll weevil habitat is not established.

Recent rains have spurred the recovery of rangelands and pastures and with lush new grass and in the fall comes the potential of a forage devouring pest - the fall armyworm. Pastures should be watched closely now for infestations of fall armyworms as they may infest pastures until the first frost in the fall. Larvae of the fall armyworm range from pale green to brown or black and are often striped with white to yellowish lines from head to tail. These stripes extend to the head, where they form an inverted “Y,” a distinguishing characteristic of this species. Moths are mostly black with white markings on the wings.

Two species of armyworms can damage pastures and small grains, the armyworm (Pseudaletia unipuncta) and the fall armyworm (Spodoptera frugiperda). These two species take approximately 4-5 weeks to go through the larval stage. When the worms are small, little forage is consumed. However, when the larvae reach 3/4 to 1 inch in length, they eat much more forage. It is estimated that mature worms will consume 75 to 80 percent of their entire diet in the last few days of their existence as larvae.

Armyworms should be controlled when they occur in large numbers or plant damage is becoming excessive. The fall armyworm outbreaks usually occur late in the summer and early fall. Preventive treatments normally are not justified because attacks are sporadic and egg mortality is usually high. A variety of natural enemies keep fall armyworm larvae down to moderate numbers. Early detection of larvae is the best management tool and is achieved by frequent, thorough inspection of plants. Outbreaks seem to occur shortly after a rain or supplemental irrigation.

Fall armyworms feed any time of the day or night, but are most active early in the morning or late in the evening. Susceptible fields or lawns should be scouted by counting the number of armyworms in a square foot area in eight different sites. Divide the total worm count by eight to find the average number of armyworms per square foot. Be sure to take samples in the interior of the field because this pest is often heaviest near the field margins. Sometimes, only the field margins require treatment. Thresholds in improved pastures and lawns vary with conditions but treatment should be considered when counts average three or more worms per square foot.